Imported from JiayiV/question_mixing (skills/agent-winformsexpert/SKILL.md). Install upstream with npx skills add JiayiV/question_mixing --skill agent-winformsexpert. Copyright stays with the author.

WinForms Development Guidelines

These are the coding and design guidelines and instructions for WinForms Expert Agent development. When customer asks/requests will require the creation of new projects

New Projects:

• Prefer .NET 10+. Note: MVVM Binding requires .NET 8+.
• Prefer Application.SetColorMode(SystemColorMode.System); in Program.cs at application startup for DarkMode support (.NET 9+).
• Make Windows API projection available by default. Assume 10.0.22000.0 as minimum Windows version requirement.

    <TargetFramework>net10.0-windows10.0.22000.0</TargetFramework>

Critical:

📦 NUGET: New projects or supporting class libraries often need special NuGet packages. Follow these rules strictly:

• Prefer well-known, stable, and widely adopted NuGet packages - compatible with the project's TFM.
• Define the versions to the latest STABLE major version, e.g.: [2.*,)

⚙️ Configuration and App-wide HighDPI settings: app.config files are discouraged for configuration for .NET. For setting the HighDpiMode, use e.g. Application.SetHighDpiMode(HighDpiMode.SystemAware) at application startup, not app.config nor manifest files.

Note: SystemAware is standard for .NET, use PerMonitorV2 when explicitly requested.

VB Specifics:

• In VB, do NOT create a Program.vb - rather use the VB App Framework.
• For the specific settings, make sure the VB code file ApplicationEvents.vb is available. Handle the ApplyApplicationDefaults event there and use the passed EventArgs to set the App defaults via its properties.

🎯 Critical Generic WinForms Issue: Dealing with Two Code Contexts

Decision: In .designer.cs or InitializeComponent → Designer rules. Otherwise → Modern C# rules.

🚨 Designer File Rules (TOP PRIORITY)

⚠️ Make sure Diagnostic Errors and build/compile errors are eventually completely addressed!

❌ Prohibited in InitializeComponent

Allowed method calls: Designer-supporting interface methods like SuspendLayout, ResumeLayout, BeginInit, EndInit

❌ Prohibited in .designer.cs File

❌ Method definitions (except InitializeComponent, Dispose, preserve existing additional constructors)
❌ Properties
❌ Lambda expressions, DO ALSO NOT bind events in InitializeComponent to Lambdas! ❌ Complex logic ❌ ??/?./?[] (null coalescing/conditional), nameof() ❌ Collection Expressions

✅ Correct Pattern

✅ File-scope namespace definitions (preferred)

📋 Required Structure of InitializeComponent Method

(Try meaningful naming of controls, derive style from existing codebase, if possible.)

private void InitializeComponent()
{
    // 1. Instantiate
    _picDogPhoto = new PictureBox();
    _lblDogographerCredit = new Label();
    _btnAdopt = new Button();
    _btnMaybeLater = new Button();
    
    // 2. Components
    components = new Container();
    
    // 3. Suspend
    ((ISupportInitialize)_picDogPhoto).BeginInit();
    SuspendLayout();
    
    // 4. Configure controls
    _picDogPhoto.Location = new Point(12, 12);
    _picDogPhoto.Name = "_picDogPhoto";
    _picDogPhoto.Size = new Size(380, 285);
    _picDogPhoto.SizeMode = PictureBoxSizeMode.Zoom;
    _picDogPhoto.TabStop = false;
    
    _lblDogographerCredit.AutoSize = true;
    _lblDogographerCredit.Location = new Point(12, 300);
    _lblDogographerCredit.Name = "_lblDogographerCredit";
    _lblDogographerCredit.Size = new Size(200, 25);
    _lblDogographerCredit.Text = "Photo by: Professional Dogographer";
    
    _btnAdopt.Location = new Point(93, 340);
    _btnAdopt.Name = "_btnAdopt";
    _btnAdopt.Size = new Size(114, 68);
    _btnAdopt.Text = "Adopt!";

    // OK, if BtnAdopt_Click is defined in main .cs file
    _btnAdopt.Click += BtnAdopt_Click;
    
    // NOT AT ALL OK, we MUST NOT have Lambdas in InitializeComponent!
    _btnAdopt.Click += (s, e) => Close();
    
    // 5. Configure Form LAST
    AutoScaleDimensions = new SizeF(13F, 32F);
    AutoScaleMode = AutoScaleMode.Font;
    ClientSize = new Size(420, 450);
    Controls.Add(_picDogPhoto);
    Controls.Add(_lblDogographerCredit);
    Controls.Add(_btnAdopt);
    Name = "DogAdoptionDialog";
    Text = "Find Your Perfect Companion!";
    ((ISupportInitialize)_picDogPhoto).EndInit();
    
    // 6. Resume
    ResumeLayout(false);
    PerformLayout();
}

#endregion

// 7. Backing fields at EOF

private PictureBox _picDogPhoto;
private Label _lblDogographerCredit;
private Button _btnAdopt;

Remember: Complex UI configuration logic goes in main .cs file, NOT .designer.cs.

Modern C# Features (Regular Code Only)

Apply ONLY to .cs files (event handlers, business logic). NEVER in .designer.cs or InitializeComponent.

Style Guidelines

Property Patterns (⚠️ CRITICAL - Common Bug Source!)

// ❌ WRONG - Memory leak
public Brush BackgroundBrush => new SolidBrush(BackColor);

// ✅ CORRECT - Cached
public Brush BackgroundBrush { get; } = new SolidBrush(Color.White);

// ✅ CORRECT - Dynamic
public Font CurrentFont => _customFont ?? DefaultFont;

Never "refactor" one to another without understanding semantic differences!

Prefer Switch Expressions over If-Else Chains

// ✅ NEW: Instead of countless IFs:
private Color GetStateColor(ControlState state) => state switch
{
    ControlState.Normal => SystemColors.Control,
    ControlState.Hover => SystemColors.ControlLight,
    ControlState.Pressed => SystemColors.ControlDark,
    _ => SystemColors.Control
};

Prefer Pattern Matching in Event Handlers

// Note nullable sender from .NET 8+ on!
private void Button_Click(object? sender, EventArgs e)
{
    if (sender is not Button button || button.Tag is null)
        return;
    
    // Use button here
}

When designing Form/UserControl from scratch

File Structure

Main file: Logic and event handlers
Designer file: Infrastructure, constructors, Dispose, InitializeComponent, control definitions

C# Conventions

• File-scoped namespaces
• Assume global using directives
• NRTs OK in main Form/UserControl file; forbidden in code-behind .designer.cs
• Event handlers: object? sender
• Events: nullable (EventHandler?)

VB.NET Conventions

• Use Application Framework. There is no Program.vb.
• Forms/UserControls: No constructor by default (compiler generates with InitializeComponent() call)
• If constructor needed, include InitializeComponent() call
• CRITICAL: Friend WithEvents controlName as ControlType for control backing fields.
• Strongly prefer event handlers Subs with Handles clause in main code over AddHandler in fileInitializeComponent

Classic Data Binding and MVVM Data Binding (.NET 8+)

Breaking Changes: .NET Framework vs .NET 8+

Data Binding Rules

• Object DataSources: INotifyPropertyChanged, BindingList<T> required, prefer ObservableObject from MVVM CommunityToolkit.
• ObservableCollection<T>: Requires BindingList<T> a dedicated adapter, that merges both change notifications approaches. Create, if not existing.
• One-way-to-source: Unsupported in WinForms DataBinding (workaround: additional dedicated VM property with NO-OP property setter).

Add Object DataSource to Solution, treat ViewModels also as DataSources

To make types as DataSource accessible for the Designer, create .datasource file in Properties\DataSources\:

<?xml version="1.0" encoding="utf-8"?>
<GenericObjectDataSource DisplayName="MainViewModel" Version="1.0" 
    xmlns="urn:schemas-microsoft-com:xml-msdatasource">
  <TypeInfo>MyApp.ViewModels.MainViewModel, MyApp.ViewModels, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null</TypeInfo>
</GenericObjectDataSource>

Subsequently, use BindingSource components in Forms/UserControls to bind to the DataSource type as "Mediator" instance between View and ViewModel. (Classic WinForms binding approach)

New MVVM Command Binding APIs in .NET 8+

Note: ToolStripItem now derives from BindableComponent.

MVVM Pattern in WinForms (.NET 8+)

• If asked to create or refactor a WinForms project to MVVM, identify (if already exists) or create a dedicated class library for ViewModels based on the MVVM CommunityToolkit

• Reference MVVM ViewModel class library from the WinForms project

• Import ViewModels via Object DataSources as described above

• Use new Control.DataContext for passing ViewModel as data sources down the control hierarchy for nested Form/UserControl scenarios

• Use Button[Base].Command or ToolStripItem.Command for MVVM command bindings. Use the CommandParameter property for passing parameters.

• • Use the Parse and Format events of Binding objects for custom data conversions (IValueConverter workaround), if necessary.

private void PrincipleApproachForIValueConverterWorkaround()
{
   // We assume the Binding was done in InitializeComponent and look up 
   // the bound property like so:
   Binding b = text1.DataBindings["Text"];

   // We hook up the "IValueConverter" functionality like so:
   b.Format += new ConvertEventHandler(DecimalToCurrencyString);
   b.Parse += new ConvertEventHandler(CurrencyStringToDecimal);
}

• Bind property as usual.
• Bind commands the same way - ViewModels are Data SOurces! Do it like so:

// Create BindingSource
components = new Container();
mainViewModelBindingSource = new BindingSource(components);

// Before SuspendLayout
mainViewModelBindingSource.DataSource = typeof(MyApp.ViewModels.MainViewModel);

// Bind properties
_txtDataField.DataBindings.Add(new Binding("Text", mainViewModelBindingSource, "PropertyName", true));

// Bind commands
_tsmFile.DataBindings.Add(new Binding("Command", mainViewModelBindingSource, "TopLevelMenuCommand", true));
_tsmFile.CommandParameter = "File";

WinForms Async Patterns (.NET 9+)

Control.InvokeAsync Overload Selection

⚠️ Fire-and-Forget Trap

// ❌ WRONG - Analyzer violation, fire-and-forget
await InvokeAsync<string>(() => await LoadDataAsync());

// ✅ CORRECT - Use async overload
await InvokeAsync<string>(async (ct) => await LoadDataAsync(ct), outerCancellationToken);

Form Async Methods (.NET 9+)

• ShowAsync(): Completes when form closes. Note that the IAsyncState of the returned task holds a weak reference to the Form for easy lookup!
• ShowDialogAsync(): Modal with dedicated message queue

CRITICAL: Async EventHandler Pattern

• All the following rules are true for both [modifier] void async EventHandler(object? s, EventArgs e) as for overridden virtual methods like async void OnLoad or async void OnClick.
• async void event handlers are the standard pattern for WinForms UI events when striving for desired asynch implementation.
• CRITICAL: ALWAYS nest await MethodAsync() calls in try/catch in async event handler — else, YOU'D RISK CRASHING THE PROCESS.

Exception Handling in WinForms

Application-Level Exception Handling

WinForms provides two primary mechanisms for handling unhandled exceptions:

AppDomain.CurrentDomain.UnhandledException:

• Catches exceptions from any thread in the AppDomain
• Cannot prevent application termination
• Use for logging critical errors before shutdown

Application.ThreadException:

• Catches exceptions on the UI thread only
• Can prevent application crash by handling the exception
• Use for graceful error recovery in UI operations

Exception Dispatch in Async/Await Context

When preserving stack traces while re-throwing exceptions in async contexts:

try
{
    await SomeAsyncOperation();
}
catch (Exception ex)
{
    if (ex is OperationCanceledException)
    {
        // Handle cancellation
    }
    else
    {
        ExceptionDispatchInfo.Capture(ex).Throw();
    }
}

Important Notes:

• Application.OnThreadException routes to the UI thread's exception handler and fires Application.ThreadException.
• Never call it from background threads — marshal to UI thread first.
• For process termination on unhandled exceptions, use Application.SetUnhandledExceptionMode(UnhandledExceptionMode.ThrowException) at startup.
• VB Limitation: VB cannot await in catch block. Avoid, or work around with state machine pattern.

CRITICAL: Manage CodeDOM Serialization

Code-generation rule for properties of types derived from Component or Control:

public class CustomControl : Control
{
    private Font? _customFont;
    
    // Simple default - no serialization if default
    [DefaultValue(typeof(Color), "Yellow")]
    public Color HighlightColor { get; set; } = Color.Yellow;
    
    // Hidden - never serialize
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
    public List<string> RuntimeData { get; set; }
    
    // Conditional serialization
    public Font? CustomFont
    {
        get => _customFont ?? Font;
        set { /* setter logic */ }
    }
    
    private bool ShouldSerializeCustomFont()
        => _customFont is not null && _customFont.Size != 9.0f;
    
    private void ResetCustomFont()
        => _customFont = null;
}

Important: Use exactly ONE of the above approaches per property for types derived from Component or Control.

WinForms Design Principles

Core Rules

Scaling and DPI:

• Use adequate margins/padding; prefer TableLayoutPanel (TLP)/FlowLayoutPanel (FLP) over absolute positioning of controls.

• The layout cell-sizing approach priority for TLPs is:

  • Rows: AutoSize > Percent > Absolute
  • Columns: AutoSize > Percent > Absolute

• For newly added Forms/UserControls: Assume 96 DPI/100% for AutoScaleMode and scaling

• For existing Forms: Leave AutoScaleMode setting as-is, but take scaling for coordinate-related properties into account

• Be DarkMode-aware in .NET 9+ - Query current DarkMode status: Application.IsDarkModeEnabled

  • Note: In DarkMode, only the SystemColors values change automatically to the complementary color palette.

• Thus, owner-draw controls, custom content painting, and DataGridView theming/coloring need customizing with absolute color values.

Layout Strategy

Divide and conquer:

• Use multiple or nested TLPs for logical sections - don't cram everything into one mega-grid.
• Main form uses either SplitContainer or an "outer" TLP with % or AutoSize-rows/cols for major sections.
• Each UI-section gets its own nested TLP or - in complex scenarios - a UserControl, which has been set up to handle the area details.

Keep it simple:

• Individual TLPs should be 2-4 columns max
• Use GroupBoxes with nested TLPs to ensure clear visual grouping.
• RadioButtons cluster rule: single-column, auto-size-cells TLP inside AutoGrow/AutoSize GroupBox.
• Large content area scrolling: Use nested panel controls with AutoScroll-enabled scrollable views.

Sizing rules: TLP cell fundamentals

• Columns:

  • AutoSize for caption columns with Anchor = Left | Right.
  • Percent for content columns, percentage distribution by good reasoning, Anchor = Top | Bottom | Left | Right. Never dock cells, always anchor!
  • Avoid Absolute column sizing mode, unless for unavoidable fixed-size content (icons, buttons).

• Rows:

  • AutoSize for rows with "single-line" character (typical entry fields, captions, checkboxes).
  • Percent for multi-line TextBoxes, rendering areas AND filling distance filler for remaining space to e.g., a bottom button row (OK|Cancel).
  • Avoid Absolute row sizing mode even more.

• Margins matter: Set Margin on controls (min. default 3px).

• Note: Padding does not have an effect in TLP cells.

Common Layout Patterns

Single-line TextBox (2-column TLP)

Most common data entry pattern:

• Label column: AutoSize width
• TextBox column: 100% Percent width
• Label: Anchor = Left | Right (vertically centers with TextBox)
• TextBox: Dock = Fill, set Margin (e.g., 3px all sides)

Multi-line TextBox or Larger Custom Content - Option A (2-column TLP)

• Label in same row, Anchor = Top | Left
• TextBox: Dock = Fill, set Margin
• Row height: AutoSize or Percent to size the cell (cell sizes the TextBox)

Multi-line TextBox or Larger Custom Content - Option B (1-column TLP, separate rows)

• Label in dedicated row above TextBox
• Label: Dock = Fill or Anchor = Left
• TextBox in next row: Dock = Fill, set Margin
• TextBox row: AutoSize or Percent to size the cell

Critical: For multi-line TextBox, the TLP cell defines the size, not the TextBox's content.

Container Sizing (CRITICAL - Prevents Clipping)

For GroupBox/Panel inside TLP cells:

• MUST set AutoSize = true and AutoSizeMode = GrowOnly
• Should Dock = Fill in their cell
• Parent TLP row should be AutoSize
• Content inside GroupBox/Panel should use nested TLP or FlowLayoutPanel

Why: Fixed-height containers clip content even when parent row is AutoSize. The container reports its fixed size, breaking the sizing chain.

Modal Dialog Button Placement

Pattern A - Bottom-right buttons (standard for OK/Cancel):

• Place buttons in FlowLayoutPanel: FlowDirection = RightToLeft
• Keep additional Percentage Filler-Row between buttons and content.
• FLP goes in bottom row of main TLP
• Visual order of buttons: [OK] (left) [Cancel] (right)

Pattern B - Top-right stacked buttons (wizards/browsers):

• Place buttons in FlowLayoutPanel: FlowDirection = TopDown
• FLP in dedicated rightmost column of main TLP
• Column: AutoSize
• FLP: Anchor = Top | Right
• Order: [OK] above [Cancel]

When to use:

• Pattern A: Data entry dialogs, settings, confirmations
• Pattern B: Multi-step wizards, navigation-heavy dialogs

Complex Layouts

• For complex layouts, consider creating dedicated UserControls for logical sections.
• Then: Nest those UserControls in (outer) TLPs of Form/UserControl, and use DataContext for data passing.
• One UserControl per TabPage keeps Designer code manageable for tabbed interfaces.

Modal Dialogs

Use DataContext property (.NET 8+) of Form to pass and return modal data objects.

Layout Recipes

Accessibility

• CRITICAL: Set AccessibleName and AccessibleDescription on actionable controls
• Maintain logical control tab order via TabIndex (A11Y follows control addition order)
• Verify keyboard-only navigation, unambiguous mnemonics, and screen reader compatibility

TreeView and ListView

DataGridView

• Prefer derived class with double buffering enabled
• Configure colors when in DarkMode!
• Large data: page/virtualize (VirtualMode = True with CellValueNeeded)

Resources and Localization

• String literal constants for UI display NEED to be in resource files.
• When laying out Forms/UserControls, take into account that localized captions might have different string lengths.
• Instead of using icon libraries, try rendering icons from the font "Segoe UI Symbol".
• If an image is needed, write a helper class that renders symbols from the font in the desired size.

Critical Reminders